The invention relates to a device and a method for use in wavelength division multiplexed (WDM) systems, and more particularly, to a device and a method for optical power equalization in WDM systems.
Many fiber optic communications systems have been commercialized in which multiple wavelengths are used to carry the signals, thus increasing capacity. These are referred to as wavelength division multiplexed (WDM) systems.
In a typical lightwave WDM system, especially those operating over long distances, the power level of the signal changes as a function of wavelength. This causes some channels to have higher power levels than others, and is known in the art as tilt. Excessive tilt in the power spectrum is a serious disadvantage since the individual channels must be compensated for unequal power levels, which raises the cost of the overall system. Techniques for overcoming tilt in WDM systems have been devised.
For example, Moving Anti-Reflection Switch (MARS) devices have been proposed as power equalizers. The basic MARS device structure and operation are well known. The MARS device typically has a movable conductive membrane suspended over a conductive substrate. With an appropriate electrostatic field, the membrane can be controllably moved towards, or away from, the substrate, thus producing a precisely controlled air gap between the membrane and the substrate. With proper positioning of the membrane with respect to the substrate, the MARS device can be switched from a reflecting state to an anti-reflecting state. Additional details of the MARS device are described in K. W. Goossen, J. A. Walker, and S. C. Arney, xe2x80x9cSilicon Modulator based on Mechanically-Active Anti-Reflection Layer with 1 Mbit/sec Capability for Fiber-in-the-Loop Applicationsxe2x80x9d, IEEE Phot. Tech. Lett., vol. 6, pp. 1119-1121, September 1994; while the basic structure and properties of a MARS device with a movable membrane are also described in U.S. Pat. No. 5,751,469, issued May 12, 1998, both of which are incorporated herein by reference.
According to this approach, two or more MARS devices, one having positive tilt and the other having negative tilt are used. By adjusting air gaps of the two devices, power equalization can be obtained over the WDM band of the system.
While this approach to power equalization is effective, other solutions may provide more design options and possibly simpler devices.
Embodiments of the invention provide a device and a method for modulating an optical signal. The device comprises a multi-layer structure incorporating at least two gaps with variable spacings. The gap spacings can be varied by electrically biasing a movable membrane in the device, resulting in a change in optical response, e.g., reflectivity, of the device. Such a device can be used as an optical power equalizer in a wavelength division multiplexed system.